As techniques for transmitting information wirelessly proliferate and develop, the potential for conflicts may arise, particularly with regard to the use of similar portions of the radio frequency spectrum by disparate systems. For example, Wireless Local Area Network (WLAN) devices, such as those conforming to Institute for Electrical and Electronic Engineers (IEEE) 802.11 protocols, using the 5 GHz frequency band may interfere with radar systems using similar frequencies. To minimize conflicts, a wireless communications device may be configured to avoid operating on frequencies in which radar signals have been detected. For example, a number of regulatory domains, including in the United States (governed by the Federal Communications Commission, FCC), in Europe (governed by the European Telecommunications Standards Institute, ETSI) and in Japan have established requirements that in order to use certain 5 GHz frequencies, the WLAN systems must be capable of Dynamic Frequency Selection (DFS.) Generally, a DFS capable master device monitors the spectrum and selects a frequency for operation that is not already in use by a radar system. Further, the master device, such as an access point (AP) in WLAN systems, may be required to continually monitor the radio environment for radar presence. When radar use within the frequency band is detected, the AP may be required to cease all transmissions within a designated time period and dynamically recommence operation on another channel.
Radar pulses are usually narrowband and have a fixed frequency and many radar signals in the 5 GHz spectrum typically include periodic bursts of radar pulses. The bursts typically have a period of about 1 ms and the pulse duration is typically between 1-5 μs, although longer pulse durations of 50-100 μs are also possible. Other radar signals, such as those employed by weather stations or the military may also exhibit a sweep signal, also known as a chirp pulse, where the frequency of the signal varies over time within a fixed bandwidth. As such, detection of signals having these characteristics is also desirable.
One technique for detecting the presence of radar operation may involve spectrally analyzing the signal received by a wireless communications device to identify a narrowband pattern that is distinguishable from a wideband pattern associated with a WLAN transmission. When a signal having a strength exceeding a threshold is detected that does not span too great a frequency range, it may be classified as a radar detection and triggers the channel switching requirements of the DFS system. In some situations, however, this technique may result in a false positive detection due to characteristics of other signals that may appear to match the narrowband characteristics of a radar signal. For example, false detection may result from in band traffic, particularly in a frequency selective channel conditions, from adjacent band traffic, from alternate far blockers aliasing in band, or in other situations.
The ramifications of a positive radar detection may be significant. As noted above, DFS requirements may specify that a wireless communications device vacate a channel on which radar has been detected for a significant period of time, such as up to 30 minutes in some regulatory domains. In turn, this may result in an interruption in WLAN operation and reduced throughput. These problems may be exacerbated in applications involving higher bandwidth modes of operation, such as those using 80 MHz channels, as relatively few channels are available in the 5 GHz spectrum. Accordingly, it would be desirable to provide systems and methods for detecting radar signals that identify false detections so that WLAN operation is not needlessly compromised. This disclosure satisfies this and other goals.